All kinds of silver wires are produced from the volatiles emitted from the front end of the flow material. For example, the degradation silver wire refers to the partial degradation of the thermoplastic after heating, and the formation of small bubbles on the surface of the plastic part when the gas decomposes. The traces left by these small bubbles on the surface of the plastic part are generally arranged in a "V" shape, and the tip of the "V" character is back to the center of the gate.For example, the main reason for the production of water vapor silver wire is that the water content in the raw material is too high, and the bubbles generated during the evaporation of water lead to the production of silver wire on the surface of the plastic part, especially the super absorbent resin such as polyamide and polyester. If the gas generated by the evaporation of water in the melt cannot be completely discharged, the water vapor silver wire will be formed on the surface of the plastic part.In the process of mold filling, the interference of gas often causes silver streaks or tiny bubbles on the surface of products or bubbles in the thick wall of products.
The main source of these gases is that the raw materials contain excessive moisture or volatile substances or lubricants, or the degradation of plastics due to high material temperature and long heating time or poor exhaust gas.1, Equipment:The nozzle hole is too small, the material is slobbering or drawing at the nozzle, the barrel or nozzle has obstacles, and the high-speed material flow generates friction heat to decompose the material.2, Mold:Because of the design defects, such as: gate position is not good, gate is too small, gate arrangement of multi gate parts is asymmetric, flow channel is small, mold cooling system is unreasonable and the mold temperature difference is too large, the molten material flow in the mold cavity is not continuous, and the air channel is blocked.
The air in the mold cavity can not leave at the same time when the plastic enters due to the lack of necessary exhaust channels or insufficient exhaust channels, blockage, poor position, and no processing gaps such as inserts and ejector pins.The surface roughness of the die is poor and the friction resistance is large, which causes local over hot spots and decomposes the passing plastic.3, Process:The material temperature is too high, causing decomposition. If the barrel temperature is too high or the heating is out of order, the barrel temperature shall be reduced one by one.The temperature of the feeding section is too high, which makes part of the plastic melt prematurely and fill the screw groove, and the air cannot be discharged from the feeding port.
The injection speed is too fast, which causes the molten plastic to decompose under the action of large shear and generates decomposition gas; the injection speed is too slow to fill the mold cavity in time, resulting in insufficient density of the product surface and craze. To reduce craze with multi-stage injection: medium speed injection to fill the runner → slow filling the gate → fast injection → low pressure and slow filling the mold, so that the gas in the mold can be eliminated in each section in time.The flow and forming pressure of the melted material will be affected by insufficient material, too large feeding buffer, too low material temperature or too low mold temperature, which will produce bubbles.When the screw is pre molded, the back pressure is too low and the rotation speed is too high, which makes the screw return too fast, and the air is easy to push to the front end of the barrel together with the material. The melting material has poor plasticization. If the melting material is not heated enough in the barrel and the plasticization is not good, the incomplete melting material particles will be exposed on the surface of the plastic part to form the stripes.
This kind of dark spot is like mica sheet, and the area of each dark spot is close to the size of a grain.Under the transmission light, it can be seen that the transparency of the stripes is poor, sometimes it can clearly distinguish the protuberant raw material particles. According to the causes of dark spots, when removing this fault, it is necessary to properly increase the barrel temperature and extend the molding cycle, and try to use the internal heating injection port or increase the cold cavity and lengthen the flow channel. If possible, the injection molding machine with large barrel length diameter ratio can also be used to enhance the plasticization of melted materials.4, Raw materials:When a large amount of powder is mixed in the raw materials with different plastics or granular materials, air is easily entrained during melting, and sometimes craze appears. Raw materials are easy to decompose when th
